The present invention relates generally to the field of circuit breakers and more specifically to sensing ground fault and individual phases in a circuit breaker.
Circuit breakers monitor a number of power conditions between a source and a load. While household circuit breakers sometimes only monitor overload conditions, industrial circuit breakers typically monitor overcurrent, short circuit, and ground fault conditions.
The total current flowing from the source through the circuit breaker to the load should be equal to the total current returning from the load through the circuit breaker to the source. A ground fault occurs when the current path between the load and the source is misdirected due to an anomaly in the circuit path. As a result, some of the current that flows through the circuit breaker to the load does not return through the normal ground path because it leaks to ground or returns along a different path than the rest of the current. For example, the current may leak to ground along the chassis of a motor. Ground faults can adversely affect the operation of appliances, or other electrical equipment, as well as the electrical network. If a ground fault occurs in a circuit supplying current to the three phase motor, the motor may begin vibrating because it is not receiving even levels of current from each phase.
Several methods of detecting a ground fault have been disclosed in the prior art. One method includes monitoring each of the phases in the distribution circuit using a microprocessor, converting sense signals representative of the power signal to digital form computationally, and summing the digitized signals to determine whether any imbalance exists among the signals. However, this method can introduce digitization errors into the signals which are cumulative as the power signals are summed. Furthermore, this method is complex and requires expensive circuitry and software programming to implement.
Accordingly, an improved method of and apparatus for sensing ground faults is needed which is simpler, more accurate and less expensive than prior methods. Further, there is a need for an efficient ground fault detection circuit which is not susceptible to digitization errors.
One embodiment relates to an apparatus for differentially sensing a ground fault in a power distribution circuit configured to route a plurality of power signals, the power signals when summed resulting in a zero sum signal. The apparatus includes a circuit element, first and second current transformers (CTs) and a routing circuit. The first and second current transformers are configured to sense first and second power signals and to generate first and second sense signals representative of the first and second power signals, respectively. The routing circuit is configured to receive the first and second sense signals and to provide a residual signal across the circuit element.
Another embodiment relates to a circuit breaker having a circuit for differentially sensing a ground fault in a power distribution circuit configured to route a plurality of power signals, the power signals when summed resulting in a zero sum signal. The circuit breaker includes a resistor, first means for sensing a first of the plurality of power signals and generating a first sense signal representative of the first power signal, and second means for sensing a second of the plurality of power signals and generating a second sense signal representative of the second power signal. The circuit breaker further includes a routing means for routing the first and second sense signals across the resistive element to provide a residual signal across the resistive element.
Another embodiment relates to a method for differentially sensing a ground fault in a power distribution circuit configured to route a plurality of power signals, the power signals when summed resulting in a zero sum signal. The method includes: sensing a first of the plurality of power signals; generating a first sense signal representative of the first power signal; sensing a second of the plurality of power signals; generating a second sense signal representative of the second power signal; routing the first and second sense signals across a circuit element to provide a residual signal across the circuit element.